[RFC] Run Coverity only twice per week.

Follow-up from neovim/neovim#1157: Coverity will run on Wednesdays and Saturdays only.
From @aktau's [comment](https://github.com/neovim/neovim/pull/1157#issuecomment-55155398), I thought we had only 2 Coverity tokens per week, that's why I did it like this.